A tuning fork crystal oscillator used for a vibratory gyro is manufactured by the steps of cutting a crystal oscillator piece to a desired shape from a crystal wafer, forming electrodes for causing the crystal oscillator piece to oscillate, and packaging the crystal oscillator piece with the electrodes formed thereon into a container. Of these steps, the step of cutting the crystal oscillator piece from a crystal wafer is particularly important because the shape of the crystal oscillator piece determines the motion of vibration and greatly affects the device performance.
FIG. 8 is a diagram showing crystal axes of a crystal oscillator piece.
As shown in FIG. 8, the crystal oscillator piece is formed from a Z-cut crystal wafer cut along a plane perpendicular to the Z axis of the crystal or from a crystal wafer 100 obtained by rotating through 0° to 10° about the X axis with respect to the Z-cut crystal wafer. The crystal axes of the crystal wafer, after rotation about the X axis, are X, Y′, and Z′. This means that the principal plane of the crystal wafer 100 is the X-Y′ plane.
FIG. 9 is a diagram schematically showing the crystal oscillator piece 110 cut from the crystal wafer 100.
FIG. 9(a) is a schematic plan view of the crystal oscillator piece 110, FIG. 9(b) is a diagram showing one example of a cross-sectional view taken along A-A′ in FIG. 9(a), and FIG. 9(c) is a diagram showing another example of a cross-sectional view taken along A-A′ in FIG. 9(a).
The crystal oscillator piece 110 comprises a support portion 111, a base portion 112, and vibrating tines 113. The vibrating tines 113 are the portions that vibrate. The vibrating tines 113 each have a width extending in the X axis direction, a length in the Y′ axis direction, and a thickness in the Z′ axis direction.
In the step of cutting the crystal oscillator piece 110 from the crystal wafer 100, a method utilizing photolithography and wet etching is employed because small-sized crystal oscillator pieces with good accuracy and at low cost can be mass-produced.
FIG. 10 is a diagram showing a method for manufacturing the crystal oscillator piece. FIG. 10 shows cross sections of the vibrating tines of the crystal oscillator piece.
First, as shown in FIG. 10(b), corrosion resistant metal films 200a and 200b having resistance to crystal etchants and photoresist films 201a and 201b overlying the corrosion resistant metal films 200a and 200b are formed on the upper and lower surfaces of the crystal wafer 100 prepared to a desired thickness as shown in FIG. 10(a).
Next, as shown in FIG. 10(c), the photoresist films 201a and 201b are exposed to radiation through two photomasks 205 and 206 with oscillator patterns written thereon that perfectly overlay each other when they are placed facing each other.
Next, the photoresist films 201a and 201b are developed. Then, using the thus developed photoresist patterns as masks, the corrosion resistant metal films 200a and 200b are patterned, as shown in FIG. 10(d), to form etching masks 207a and 207b for crystal etching.
Next, the remaining photoresist films 201a and 201b are removed. Thereafter, the crystal wafer 101 with the etching masks 207a and 207b formed on both surfaces thereof is immersed in an etching solution of hydrofluoric acid, and the portions of the crystal that are not covered with the etching masks 207a and 207b are dissolved from both surfaces, as shown in FIG. 10(e). After that, the etching masks 207a and 207b are removed, completing the fabrication of the crystal oscillator piece 110 such as shown in FIG. 9(a).
In another method for manufacturing of a crystal oscillator piece, an etching mask is patterned only on one surface, with the other surface completely covered with a corrosion resistant metal film, and etching is performed from the one surface (for example, refer to patent document 1).
In still another method for manufacturing of a crystal oscillator piece, as shown in FIG. 11, the etching mask pattern 207d on the lower surface is formed wider than the pattern 207c on the upper surface, and etching is performed using the pattern 207c on the upper surface as the reference pattern (for example, refer to patent document 2).
FIG. 12 is a diagram explaining the direction of vibration of the crystal oscillator piece.
FIG. 12(a) is a perspective view of the crystal oscillator piece, FIG. 12(b) is a diagram showing one example of the direction of vibration in an A-A′ cross section of FIG. 12(a), and FIG. 12(c) is a diagram showing another example of the direction of vibration in the A-A′ cross section of FIG. 12(a).
As shown in FIG. 12(a), when using the tuning fork crystal oscillator for a vibratory gyro, flexural vibration in the X axis direction is used as driving vibration, and flexural vibration in the Z′ axis direction is used as detection vibration which occurs when an angular velocity is applied. In this arrangement, in the absence of an applied angular velocity, the vibration in the Z′ axis direction should not normally occur, as shown in FIG. 12(b). However, in the tuning fork crystal oscillator manufactured by the prior art manufacturing method, there have been cases where a vibration component in the Z′ axis direction is observed, as shown in FIG. 12(c), when no angular velocity is actually applied. The vibration component in the Z′ axis direction arising from this oblique vibration is called the leakage vibration; since this vibration is indistinguishable from the detection vibration, there has been the problem that the S/N and temperature characteristics of the gyro degrade due to the leakage vibration.
In the case of tuning fork crystal oscillators for ordinary applications, the tuning fork vibration is likewise produced by using the flexural vibration in the X axis direction, and in this case also, there has been the problem that the oblique vibration containing the Z′ direction component causes the crystal impedance to rise, leading to degradation of the characteristics.
It is believed that crystal residues generated when fabricating the crystal oscillator piece by etching have some bearing on the oblique vibration. The crystal has etching anisotropy, which means that the etch rate is different in different directions of the crystal. As a result, the crystal is not uniformly etched, leaving residues on the side faces of the vibrating tines 113 of the crystal oscillator piece after etching. For example, as shown in FIGS. 9(b) and 9(b), the cross-sectional shapes of the vibrating tines of the crystal oscillator piece 110 are not precisely rectangular, but residues of triangular or other shapes are formed on the +X and −X side faces in the Y′-Z′ plane. FIG. 9(b) shows the cross-sectional shapes after etching for a short time, and FIG. 9(c) shows the cross-sectional shapes after etching for a long time.
If such residues are present, the driving vibration which should normally occur only in the X axis direction, as shown in FIG. 12(b), may be disrupted, depending on how the residues are formed, and the vibration occurs in the oblique direction by being accompanied by a component vibrating in the Z′ axis direction. This results in the generation of leakage vibration.
The leakage vibration arising from such oblique vibration tends to occur rather often when the crystal oscillator piece is manufactured by the prior art method. There has therefore developed a need to suppress the oblique vibration and reduce the leakage vibration.
Further, the relationship between the oblique vibration and the directions of the principal axes of cross section of the crystal oscillator piece is analyzed in non-patent document 1. The term “principal axes of cross section” or “principal axes” will also be used in the description given hereinafter, and it is to be understood that this terms refers to the principal axes that pass through the centroid of the vibrating tine cross section.
Patent document 1: Japanese Unexamined Patent Publication No. S52-035592 (Page 3, FIG. 4)
Patent document 2: Japanese Unexamined Patent Publication No. 2006-217497 (Page 5, FIG. 1)
Non-patent document 1: Motohiro FUJIYOSHI et al., IEICE Transactions, C Vol. J87-C, No. 9, pp. 712-719